Rekindling s -wave dark matter annihilation below 10 GeV with Breit-Wigner effects

Velocity-independent ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>s</a:mi> </a:mrow> </a:math> -wave) annihilation of thermal dark matter is ruled out by CMB data for masses below <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mo>∼</c:mo> <c:mn>10</c:mn> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>GeV</c:mi> </c:math> , effectively ruling out the possibility of indirectly detecting it in this mass range. We demonstrate in a model-independent framework that Breit-Wigner effects from very narrow resonances can circumvent CMB constraints, thereby reviving the potential to detect <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:mi>s</e:mi> </e:mrow> </e:math> -wave DM annihilation in the present Universe. The density of resonant <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mrow> <g:mi>s</g:mi> </g:mrow> </g:math> -wave dark matter continues to evolve long after chemical decoupling, leading to a scenario we refer to as , where kinetic decoupling plays a significant role in determining the relic density.